1. Field of the Invention
The present invention relates to an apparatus for measuring the weight of a vehicle seat including the weight of a passenger sitting thereon and, more particularly, to a seat weight measuring apparatus that is both highly durable and inexpensive.
2. Description of Related Art
Automobiles are equipped with seat belts and airbags to secure safety for passengers. In recent years, there is a trend for controlling the operation of such safety devices according to the weight (body weight) of a passenger for improved performance of seat belts and airbags. For example, the amount of gas to be introduced into the airbag, an airbag inflating speed, or a pre-tensioning of the seat belt may be controlled depending on the weight of a passenger. To do so, a device is needed to measure the weight of the passenger sitting on the seat. One such device is disclosed in Japanese Patent Applications No. H9-156666 and No. H10-121627, which involve arranging load sensors (load cells) at four corners of seat rails, obtaining vertical load acting on the load cells as voltage by an electric circuit, and summing them to determine the seat weight including the weight of the passenger.
The load sensors of the seat weight measuring apparatus described above are preferably of a small-sized type with a measuring capacity about 50 kg. Such load sensors may include: sensors having a strain gauge attached to (or formed on) a sensor plate that flexes when it is subjected to a load; piezoelectric type sensors; and capacitance type sensors which detect displacements of an elastic member that flexes upon receiving a load.
A metallic thin film gauge is generally employed as the strain gauge. The method of manufacturing the metallic thin film gauge is as follows: a metallic thin film, which has been attached to a resin film is etched to form a gauging portion composed of wirings and thin lines; then, the gauging portion is sandwiched between resin films and then mounted to a spring steel. The temperature coefficient (coefficient representing the relation between temperature and linear expansion) between the spring steel and the metallic thin film is adjusted to obtain a load sensor having excellent strain characteristics and temperature coefficient.
The aforementioned strain gauge, however, is costly to manufacture due to the complex manufacturing process. Further, a lot of man-hours are required to bond the strain gauge accurately and uniformly, thereby further increasing the assembly cost. Moreover, because the voltage output by the strain gauge is low, a noise shield and a sensitive amplifier are also required, which results in additional costs.
The gauging portion consisting of metallic wirings and thin lines is sandwiched between resin films to improve its heat resistance (generally from xe2x88x9235xc2x0 C. to +80xc2x0C.) and its corrosion resistance. A sufficient additional seal is required to avoid the possibility of an electrical insulation defect, which occurs under a severe working condition such as the condition inside a vehicle, i.e. high-temperature and high-humidity.
The above problems with the known apparatuses are not intended to be exhaustive, but are among many tending to reduce the effectiveness of those known apparatuses. Other noteworthy problems may exist; however, those presented above should be sufficient to demonstrate that the known apparatuses will admit to worthwhile improvement.
It is an object of the invention to provide a seat weight measuring apparatus that alleviates the aforementioned difficulties.
It is another object of the invention to provide a seat weight measuring apparatus for measuring the weight of the vehicle seat including the weight of a passenger sitting thereon, having a reduced working cost and assembly cost.
It is another object of the invention to provide a seat weight measuring apparatus having improved heat resistance and corrosion resistance.
A preferred embodiment of the invention that is intended to accomplish at least some of the foregoing objects includes a seat weight measuring apparatus for measuring a weight of the vehicle seat including the weight of a passenger sitting thereon. The apparatus includes a load sensor that receives at least a part of the seat weight and converts the seat weight into an electrical signal. The load sensor includes a sensor member which is elastically deformable upon receiving at least a part of the seat weight, and a strain gauge arranged on a surface of the sensor member. The strain gauge includes a lower insulating layer, a wiring layer, a resistor layer, and an upper insulating layer sequentially formed on the sensor member.
Additional objects and advantages of the invention will be set forth in the following description, and in part may be obvious by practicing the invention. The objects and advantages of the invention may be realized and obtained through the instrumentalities and combinations pointed out in the appended claims.